This invention relates to the interconnection of communications networks.
A communications network interconnects a number of stations to allow them to communicate with one another. It is known that such networks may themselves be interconnected by devices sometimes referred to as "bridges" or "gateways" to form a combined network in which station on either of the original networks may communicate with stations on the other network. These interconnection devices are of the store-and-forward type, and transmit messages received from one network on to the other.
There is an especial need for such devices if the networks are local area networks using an access protocol based on carrier-sense multiple access with collision detection, (CSMA/CD) because the span of such networks is limited by the need to keep the maximum propagation delay within a prescribed limit. If two such networks of the maximum span are joined together by such an interconnection device a combined network may be formed whose total span exceeds the limit imposed on a single network.
The use of devices for this purpose is described in "Ethernet: distributed packet switching for local computer networks" by R. M. Metcalfe and D. R. Boggs, Communications of the ACM, July 1976, pages 395 to 404. This paper also points out that the device may carry out address filtering by passing packets from one network to the other only if the destination station is located in the remote network. Address filtering in this manner requires the interconnection device to hold a table or record of the addresses of all stations in the remote network so that it can forward a packet only if its destination address matches one of those held. An advantage of address filtering is that it improves efficiency by reducing the packets transmitted in each of the individual networks.
One object of the present invention is to provide an improved method of address filtering.